Petroleum refineries are comprised of numerous reaction systems for effecting conversion of hydrocarbons to a multitudinous number of products. The reactions employed in these systems are usually conducted in the presence of hydrogen and they result in either the net production of hydrogen or the net consumption of hydrogen. Net hydrogen refers to either the hydrogen which is available from a reaction system for use elsewhere or to the hydrogen which must be added to a reaction system from a source outside the reaction system. Because hydrogen is a relatively expensive substance, it has become the practice in the art of hydrocarbon conversion to supply hydrogen from reaction systems which are net producers of hydrogen to systems which are net consumers.
One of the most widely practiced hydrocarbon conversion processes is catalytic reforming. Catalytic reforming is a well established hydrocarbon conversion process employed for improving the octane quality of hydrocarbon feedstocks, the primary product of reforming being motor gasoline. Other products of catalytic reforming comprise light hydrocarbons such as propane and butane. Catalytic reforming is a net hydrogen producing reaction system. The art of catalytic reforming is well known and does not require detailed description herein beyond that required to explain the present invention.
The usual feedstock for catalytic reforming is a petroleum fraction known as naphtha and having an initial boiling point of about 180 degrees Fahrenheit (82.degree. C.) and an end boiling point of about 400 degrees Fahrenheit (204.degree. C.). Catalytic reforming is a vapor phase reaction effected at temperatures ranging from about 500 to about 1050 degrees Fahrenheit (260.degree.-566.degree. C.) and at pressures ranging from about 50 to about 1000 psig (446-6996 kPaa), preferably from about 85 to about 350 psig (687-2515 kPaa). The reaction is carried out in the presence of sufficient hydrogen to provide a hydrogen to hydrocarbon mole ratio of from about 0.5:1 to about 10:1. Further information on catalytic reforming processes may be found in, for example, U.S. Pat. Nos. 4,119,526 (Peters et al.); 4,409,095 (Peters); and 4,440,626 (Winter et al.).
Catalytic reforming is the traditional octane controller in a refinery, that is, the catalytic reforming process is adjusted to vary the octane rating of the reformate product. For example, increasing temperature in a catalytic reforming zone results in a reformate of increased octane rating. Other hydrocarbons are blended with reformate in the production of motor gasoline, but the octane rating of the refinery motor gasoline pool is determined primarily by the octane rating of reformate.
Also, lead-based compounds are added to gasoline in order to enhance octane. The government mandated reduction of lead content of "regular" gasoline and the total elimination of lead in the major portion of gasoline used in this country has caused refiners to look for methods of increasing the octane rating of reformate. One method of reformate octane rating is to increase the severity of the reaction. Increasing reaction temperature increases severity, as does lowering the pressure at which the reaction takes place. Reducing the hydrogen to hydrocarbon ratio also promotes greater severity. It should be noted that increasing the severity of the reaction results in a higher coke make, that is, a higher rate of deposition of coke on the catalyst in the reaction zone. The higher rate of deposition of coke requires more frequent catalyst regeneration.
It can be seen that severity is an umbrella term which covers changes in a number of parameters. Severity is also a relative term and not susceptible of precise definition. Much depends upon the original design of a unit. For example, a refining process operating at higher than design temperature is often said to be operating at an increased severity. Another similar unit operating at the same temperature but where that temperature is the design temperature is often not considered to be operating at higher severity. A change in catalyst or feedstock may induce one skilled in the art to refer to operation at increased severity. For example, in a catalytic reforming unit, a change from the normal feedstock to a thermally cracked naphtha feedstock would normally be considered to be a change to a higher severity operation. Many skilled in the art would consider operation of a catalytic reforming zone at a temperature above about 1020 degrees Fahrenheit (549.degree. C.) to be a high severity process. In a like manner, operating at a pressure below 100 psig (791 kPaa) might be considered to be a high severity process.
Effluent of a catalytic reforming zone, comprising hydrogen and hydrocarbon conversion reaction products, is subjected to processing steps in a separation zone in order to separate it into a hydrogen-rich gaseous stream and a liquid stream. There are numerous methods for effecting the separation; however, the first steps usually comprise cooling the effluent and passing it into a vapor-liquid equilibrium separation vessel, from which a hydrogen-rich vapor phase and a liquid hydrocarbon phase are recovered. Additional steps, often involving recontacting and absorption, may effect further purification of the hydrogen-rich stream, with hydrocarbons separated from the hydrogen-rich stream being combined with the liquid stream. Exemplary methods of processing reforming reaction zone effluent may be found in U.S. Pat. Nos. 3,520,800 (Forbes); 4,364,820 (deGraff et al.); and 4,333,818 (Coste). The hydrogen-rich gaseous stream is usually split into two portions, a first portion which is recycled back to the reaction zone and a second portion which is the net hydrogen stream available for use elsewhere in the refinery. The liquid hydrocarbon phase recovered from the separation zone is usually fed to a fractionation column whose bottoms product is reformate.
There are a number of hydrocarbon conversion reactions which take place in a catalytic reforming zone, including undesirable side reactions. Certain of these undesirable reactions result in the production of polycyclic aromatic compounds, which may be abbreviated PACs. PACs are often referred to as polynuclear aromatic compounds; the term polycyclic will be used herein as it is preferred in the art. A PAC is a hydrocarbon comprising two or more hydrocarbon rings where the rings have at least one carbon atom in common. Further information on PACs may be obtained from a book entitled, "Analytical Chemistry of Polycyclic Aromatic Compounds" (Lee et al., 1981, Academic Press).
The quantity of PACs produced in a catalytic reforming reaction zone increases as severity increases. Thus, as refiners adjust the catalytic reforming process parameters to increase octane rating, they are increasingly suffering operational and maintenance problems due to the presence of higher levels of PACs in catalytic reforming reaction zone effluent. Also, the need for higher octane reformate has resulted in the use of lower quality charge stocks for catalytic reforming. The production of PACs tends to be higher when lower quality stocks such as thermally cracked naphthas, FCC naphthas, hydrocracked naphthas, and coker naphthas are charged to a catalytic reformer. It should also be noted that PACs may be present in the feedstock.
Certainly, PACs have always been present in catalytic reformer effluents. However, the quantities present have been sufficiently small that adverse effects have not been attributed to their presence. Also, it was believed, prior to this invention, that PACs cannot be present in the vaporous products. In some cases, problems resulting from the presence of PACs have been attributed to other causes. The present invention recognizes that PACs are present in the vaporous portion of catalytic reformer effluent, that such presence is the cause of problems in both vapor and liquid phase equipment, and provides a method of dealing with PACs present in the vaporous portion of catalytic reformer effluent.
Since PACs have a high molecular weight, their presence in vapor streams is unexpected and surprising. However, PACs have caused serious problems in vapor phase equipment of catalytic reforming zones. Frequently, the problems were not recognized as being due to PACs, since their presence was unexpected. PACs are frequently seen in the reciprocating compressors which are normally used on the recycle gas stream and the net gas stream. A hard gray material is frequently found on and around the valves and inside the compression chambers. This material causes the valves to stick and prevents them from closing completely. On at least one occasion, a broken compressor shaft was attributed to the material. It is believed that this gray material is comprised of PACs and ammonium chloride salts. Chlorides are usually present in the hydrocarbons in catalytic reforming units. The feedstock to a catalytic reforming unit usually contains combined nitrogen, that is, hydrocarbons having a nitrogen atom. The nitrogen atoms are hydrogenated in the reaction zone and then combine with the chlorides. Ammonium chloride salt deposition is a common problem in a number of different hydrocarbon conversion processes. Of course, deposits of PACs which do not contain salts are also found in compressors.
The reciprocating compressors can be provided with a large amount of lubricant in order to prevent deposition of the gray solids by flushing the material away before it becomes firmly attached to the surfaces and hardens. However, this solution only partially alleviates the problems and, further, simply moves the problem downstream, that is, the deposits occur elsewhere.
In one refinery, PACs and salts in the net gas compressors have been so troublesome that three compressors are installed, each capable of handling 60% of the load. This was done in the expectation of keeping the flow at 60% of capacity when two compressors are inoperative. Normal procedure, in the absence of the problems described, is to install only two compressors.
In another instance, tubes of a heat exchanger used to heat a mixture of feedstock and recycle gas were found to be plugged with an asphalt-like substance. The composition of the plug material was found to be 87% carbon. Aromatic hydrocarbons having up to seven rings were present.
There are numerous other locations in a catalytic reforming unit and associated systems, such as the fuel gas system, where deposition of PACs are capable of causing operational, maintenance, and safety problems, The deposits may be in combination with salts and are often located on relatively cool surfaces. Also, viscous black liquids comprised of PACs have been found in vapor-containing equipment and pipes.
Since a portion of the vapor stream recovered from a catalytic reformer effluent is recycled to the reaction zone and the vapor stream contains PACs, it is believed that the concentration of PACs, in the liquid portion of the reformer effluent is increased.
The presence of PACs in reformate in concentrations above about 50 ppm (by weight) is normally unacceptable. Motor gasoline is usually tested in accordance with ASTM Standard D86. When a sufficient quantity of PACs is present in motor gasoline, the end boiling point detected by the test is too high. The temperature at which 95 percent (by volume) of the sample distills over is usually satisfactory, since the PACs concentrate in the last five percent. However, the end point is 50 to 100 degrees Fahrenheit (10.degree.-38.degree. C.) or more above the 95 percent point. This result may render the gasoline unsaleable. For example, an end point of 500 degrees Fahrenheit (260.degree. C.) is clearly unsatisfactory, while an end point of 430 degrees Fahrenheit (221.degree. C.) is acceptable.
Another problem resulting from the presence of PACs in reformate is the accumulation of a black tarry material, comprised of PACs, in locations in piping and instruments which are not swept clean by the flowing liquid.
As mentioned above, coke deposition on the catalyst is accelerated by operating at high severity conditions. Independent of this effect is an additional increase in the rate of coke deposition which is due to the presence of PACs. The PACs tend to form coke and also to drag other materials down, that is, to promote coke formation by materials other than the PACs.
Thus, it can be seen that removal of PACs in the recycle stream results in benefits in liquid handling pipelines and equipment, as well as on the vapor side.